Field of the Invention
The invention concerns a method for determining a deviation in homogeneity of a magnetic field of a magnetic resonance scanner in a direction defined by an axis, as well as a test apparatus for implementing such a method.
Description of the Prior Art
In magnetic resonance scanners, the examination subject that is to be scanned, a patient for example, is typically exposed to a relatively high basic magnetic field, of 1.5 or 3 tesla for example, with the use of a basic field magnet. In addition, at least one magnetic field gradient is applied by a gradient coil unit. Radio-frequency excitation signals (RF signals) are then transmitted via a radio-frequency antenna unit by suitable antenna equipment for the purpose of tipping the nuclear spins of specific atoms excited into resonance by the radio-frequency field through a defined flip angle with respect to the magnetic field lines of the basic magnetic field. During the resulting precession of the nuclear spins, radio-frequency signals, also known as magnetic resonance signals (MR signals), are emitted, received by suitable reception coils, and then processed further to produce image data. The quality of the image data is influenced by the homogeneity of the basic magnetic field and the at least one magnetic field gradient. If the basic magnetic field and/or the magnetic field gradient exhibit local deviations, the image data may have no MR signal or a distorted MR signal at certain positions. The homogeneity of the basic magnetic field in the examination region of interest is particularly important. The examination region of interest is the term used to denote the area of the magnetic resonance scanner in which MR signals may be acquired from the examination subject without repositioning the examination subject.
During the installation of a magnetic resonance scanner, a calibration of the basic magnetic field generated by the basic field magnet is typically carried out and, based on the results, metal, for example, is placed in the scanner such that deviations in the homogeneity of the basic magnetic field are reduced. Similarly, the homogeneity of the at least one magnetic field gradient may be checked and/or improved during installation of the magnetic resonance scanner. This process is referred to as “shimming”. A magnetic field or the local deviations thereof is or are measured by a shim device. For that purpose, data are typically acquired at a number of (preferably strictly defined) positions of the magnetic field.